L>.,s THE OASTRULA OF THE XAVllO 



it, is to be regarded as a modified portion of the ventral wall of the 

 body. The splanchnopleure also envelopes it, so that, morphologically 

 speaking, the yolk lies within the mesenteron. 



The Teleostei, so far as the first formation of the layers is concerned, 

 resemble in all essential features the Elasmobranchii, but the neuren- 

 teric canal is apparently not developed (?), owing to the obliteration 

 of the neural canal; and the roof of the segmentation cavity is formed 

 of epiblast only. 



In the preceding pages I have attempted to shew that the Am- 

 phibia, Acipenser, Petromyzon, the Elasmobranchii and the Teleostei 

 agree very closely in the mode of formation of the gastrula. The 

 unsymmetrical gastrula or pseudo-gastrula which is common to them 

 all is, 1 believe, to be explained by the form of the vertebrate body. 

 In Amphioxus, where the small amount of food-yolk present is distri- 

 buted uniformly, there is no reason why the invagination and resulting 

 gastrula should not be symmetrical. In true Vertebrates, where more 

 food-yolk is present, the shape and structure of the body render it 

 necessary for the food -yolk to be stored away on the ventral side of 

 the alimentary canal. It is this fact which causes the asymmetry of 

 the gastrula, since it is not possible for the part of the ovum, which 

 will become the ventral wall of the alimentary tract, and which is 

 loaded with food-yolk, to be invaginated in the same fashion as the 

 dorsal wall. 



Sauropsida. The comparison of the different types of the Ichtliy- 

 opsida is fairly simple, but the comparison of the Sauropsida with the 

 Ichthyopsida is a far more difficult matter. In all the Sauropsida there 

 is a large food-yolk, and the segmentation agrees closely with that 

 in the Elasmobrauchii. It might have been anticipated that the 

 resemblance would continue in the subsequent development. This 

 however is far from being the case. The medullary plate, instead of 

 lying at the edge of the blastoderm, lies in the centre, and its forma- 

 tion is preceded by that of a peculiar structure, the primitive streak, 

 which, on the formation of the medullary plate, is found to lie at the 

 hinder end of the latter and to connect it with the edge of the blasto- 

 derm. 



The possibility of a comparison between the Sauropsida and 

 the Elasmobranchii depends upon the explaiiation being possible of 

 (1) the position of the embryo near the centre of the blastoderm, 

 and (2) the nature of the primitive streak. 



The answers to these two questions are, according to my view, 

 intimately bound together. 



I consider that the embryos of the Sauropsida have come to occupy 

 a central position in the blastoderm owing to the abbreviation of a pro- 

 cess similar to that by which, in Elasmobranchii, the embryo is removed 

 from the edge of the blastoderm; and that the primitive streak repre- 

 sents the linear streak connecting the Elasmobranch embryo with the 

 edge of the blastoderm after it has become removed from its previous 



